U.S. patent application number 13/817907 was filed with the patent office on 2013-06-20 for method for forming identification marks on silicon carbide single crystal substrate, and silicon carbide single crystal substrate.
This patent application is currently assigned to HITACHI METALS ,LTD.. The applicant listed for this patent is Sadahiko Kondo. Invention is credited to Sadahiko Kondo.
Application Number | 20130157009 13/817907 |
Document ID | / |
Family ID | 46457535 |
Filed Date | 2013-06-20 |
United States Patent
Application |
20130157009 |
Kind Code |
A1 |
Kondo; Sadahiko |
June 20, 2013 |
METHOD FOR FORMING IDENTIFICATION MARKS ON SILICON CARBIDE SINGLE
CRYSTAL SUBSTRATE, AND SILICON CARBIDE SINGLE CRYSTAL SUBSTRATE
Abstract
A method for forming an identification mark on a silicon carbide
single crystal substrate according to the present invention
includes: (a) scanning a principal surface of a silicon carbide
single crystal substrate with a laser beam at a first energy
density such that a groove is formed in the principal surface of
the silicon carbide single crystal substrate, thereby forming an
identification mark which is constituted of one or more grooves in
the principal surface of the silicon carbide single crystal
substrate; and (b) scanning an inside of the groove formed in the
principal surface of the silicon carbide single crystal substrate
with a laser beam at a second energy density that is lower than the
first energy density.
Inventors: |
Kondo; Sadahiko;
(Mishima-gun, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kondo; Sadahiko |
Mishima-gun |
|
JP |
|
|
Assignee: |
HITACHI METALS ,LTD.
Minato-ku ,Tokyo
JP
|
Family ID: |
46457535 |
Appl. No.: |
13/817907 |
Filed: |
January 5, 2012 |
PCT Filed: |
January 5, 2012 |
PCT NO: |
PCT/JP2012/050080 |
371 Date: |
February 20, 2013 |
Current U.S.
Class: |
428/141 ;
264/400; 428/156 |
Current CPC
Class: |
Y10T 428/24355 20150115;
H01L 2924/0002 20130101; C30B 33/04 20130101; Y10T 428/24479
20150115; B23K 26/364 20151001; H01L 2924/0002 20130101; H01L
23/544 20130101; B41M 5/24 20130101; B41M 3/14 20130101; H01L
2924/00 20130101; C30B 33/08 20130101; B23K 26/40 20130101; B23K
2103/50 20180801 |
Class at
Publication: |
428/141 ;
264/400; 428/156 |
International
Class: |
C30B 33/08 20060101
C30B033/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 6, 2011 |
JP |
2011-001235 |
Claims
1. A method for forming an identification mark on a silicon carbide
single crystal substrate, comprising: (a) scanning a principal
surface of a silicon carbide single crystal substrate with a laser
beam at a first energy density such that a groove is formed in the
principal surface of the silicon carbide single crystal substrate,
thereby forming an identification mark which is constituted of one
or more grooves in the principal surface of the silicon carbide
single crystal substrate; and (b) scanning an inside of the groove
formed in the principal surface of the silicon carbide single
crystal substrate with a laser beam at a second energy density that
is lower than the first energy density.
2. The method of claim 1, wherein a width of the groove is not less
than 50 .mu.m, and a depth of the groove is not less than 20
.mu.m.
3. The method of claim 1 wherein, at least at a bottom surface of
an internal surface of the groove, the surface roughness Ra is not
more than 1 .mu.m.
4. The method of claim 1, further comprising (c) after step (b),
performing mechanical polishing on the principal surface of the
silicon carbide single crystal substrate.
5. The method of claim 4 wherein, after step (c), gas phase etching
is performed on the principal surface of the silicon carbide single
crystal substrate.
6. The method of claim 1, wherein the surface roughness Ra of the
principal surface of the silicon carbide single crystal substrate
is not less than 0.1 nm and not more than 2.0 nm.
7. A silicon carbide single crystal substrate which has an
identification mark on a principal surface of the silicon carbide
single crystal substrate, the identification mark being constituted
of one or more grooves, wherein a width of the groove is not less
than 50 .mu.m and less than 0.5 mm, and a depth of the groove is
not less than 20 .mu.m, and a surface roughness Ra of an internal
surface of the groove is not more than 1 .mu.m.
8. The silicon carbide single crystal substrate of claim 7, wherein
the surface roughness Ra of the principal surface is not less than
0.1 nm and not more than 2.0 nm.
9. The silicon carbide single crystal substrate of claim 7, wherein
a bottom surface of the groove is a solidified surface.
10. The silicon carbide single crystal substrate of claim 9,
wherein the bottom surface of the groove has a striped pattern.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for forming an
identification mark on a silicon carbide single crystal substrate
and particularly to a method for forming an identification mark on
a silicon carbide single crystal substrate using a laser beam.
BACKGROUND ART
[0002] The silicon carbide semiconductor has a larger dielectric
breakdown electric field, a faster saturated drift velocity of
electrons, and a greater thermal conductivity than those of the
silicon semiconductor. Thus, research and development have been
intensively carried out for realizing a power device which is
capable of a large current operation at a high temperature and at a
high speed with the use of a silicon carbide semiconductor as
compared with conventional silicon devices. Among others, motors
for use in electric motorcycles, electric vehicles, and hybrid
vehicles are AC-driven or inverter-controlled, and therefore,
development of efficient switching devices for such uses has been
receiving attention. To realize such power devices, a silicon
carbide single crystal substrate for epitaxial growth of a
high-quality silicon carbide semiconductor layer is necessary.
[0003] Demands for blue laser diodes which are used as a light
source for recording data at a high density and white diodes which
are used as a light source in place of a fluorescent lamp or an
incandescent bulb have been growing. Such light-emitting devices
are manufactured using a gallium nitride semiconductor, and in some
cases, a silicon carbide single crystal substrate is used as the
substrate for formation of a high-quality gallium nitride
semiconductor layer. Therefore, there is demand for a silicon
carbide single crystal substrate which is used as a substrate for
manufacture of a semiconductor device for which demand is expected
to undergo a large growth in the future, such as a silicon carbide
semiconductor device, a gallium nitride semiconductor device,
etc.
[0004] To a semiconductor substrate which is used for manufacture
of a semiconductor device, information for identification is
provided as an identification mark for identifying semiconductor
substrates and managing the process conditions of the manufacture
process through which they have undergone for each of the
semiconductor substrates. Usually, the identification mark has a
size which is perceivable by a human eye. However, in other cases,
the identification mark is imaged by a camera or the like and
image-processed so as to be detected by a semiconductor
manufacturing apparatus or the like.
[0005] In forming an identification mark on a semiconductor
substrate, a laser beam is usually used. The semiconductor in a
region irradiated with a laser beam is melted and evaporated,
whereby a recessed portion is formed in the surface of the
semiconductor substrate. The recessed portion constitutes an
identification mark. According to the depth of this recessed
portion, the method for forming an identification mark is generally
divided into two types. Specifically, formation of an
identification mark with a recessed portion depth of about 0.1
.mu.m to 5 .mu.m is referred to as "soft marking", and formation of
an identification mark with a recessed portion depth of about 5
.mu.m to 100 .mu.m is referred to as "hard marking". Also, in some
cases, the identification mark is constituted of a recessed portion
which is in the form of an independent dot, and in other cases, the
identification mark is constituted of one or more linear
grooves.
[0006] Silicon carbide is a new semiconductor material and has a
higher melting point and a greater hardness than other
semiconductor materials which are widely employed, such as silicon,
gallium arsenide, etc. Therefore, it is generally difficult to form
a desirable identification mark on a silicon carbide single crystal
substrate under the conditions that are suitable for formation of
an identification mark on a silicon substrate. Patent Document 1
discloses the technique of forming an identification mark which has
an excellent visibility, which is realized by irradiating a silicon
carbide single crystal substrate with pulsed laser light which has
a predetermined pulse shape such that the silicon carbide is
melted, whereby a slightly-recessed region is formed which contains
a greater amount of carbon or silicon.
CITATION LIST
Patent Literature
[0007] Patent Document 1: Japanese Laid-Open Patent Publication No.
2006-43717
SUMMARY OF INVENTION
Technical Problem
[0008] According to the method of Patent Document 1, an
identification mark which is constituted of a slightly-recessed dot
is formed. Therefore, it is inferred that, according to the method
of Patent Document 1, the identification mark which is constituted
of the dot is formed by soft marking. Formation of the
identification mark by soft marking is usually performed on a
mirror-finished semiconductor substrate in many cases. However,
since formation of the identification mark leads to formation of a
bump in the substrate, there is a problem that the flatness of the
substrate is marred.
[0009] Patent Document 1 discloses that a recessed portion in the
form of a dot which constitutes an identification mark is formed by
a region which contains a greater amount of carbon or silicon,
whereby the perceivability which is attributed to reflected light
and transmitted light is improved. However, there is a problem that
an identification mark which is constituted of a dot is
intrinsically inferior in visibility to an identification mark
which is constituted of a line. Further, the recessed portion in
the form of a dot which constitutes the identification mark has a
small size, and therefore, laser dust, such as a solidified
substance of silicon carbide melted by laser irradiation, abrasive
grains, or other minute contaminants which can be produced in the
middle of the semiconductor manufacturing process readily remain in
the recessed portion in the form of a dot. Such contaminants
remaining in the recessed portion can be a cause for contamination
of the surface of the substrate when they are separated from the
recessed portion, or a cause for formation of scars in the surface,
in a substrate manufacturing process or a semiconductor device
manufacturing process which would be performed later.
[0010] The present invention solves at least one of the above
problems which arise in the prior art. One of the objects of the
present invention is to provide a method for forming a
highly-visible identification mark on a silicon carbide single
crystal substrate.
Solution to Problem
[0011] A method for forming an identification mark on a silicon
carbide single crystal substrate according to the present invention
includes: (a) scanning a principal surface of a silicon carbide
single crystal substrate with a laser beam at a first energy
density such that a groove is formed in the principal surface of
the silicon carbide single crystal substrate, thereby forming an
identification mark which is constituted of one or more grooves in
the principal surface of the silicon carbide single crystal
substrate; and (b) scanning an inside of the groove formed in the
principal surface of the silicon carbide single crystal substrate
with a laser beam at a second energy density that is lower than the
first energy density.
[0012] In a preferred embodiment, a width of the groove is not less
than 50 .mu.m, and a depth of the groove is not less than 20
.mu.m.
[0013] In a preferred embodiment, at least at a bottom surface of
an internal surface of the groove, the surface roughness Ra is not
more than 1 .mu.m.
[0014] In a preferred embodiment, the method further includes (c)
after step (b), performing mechanical polishing on the principal
surface of the silicon carbide single crystal substrate.
[0015] In a preferred embodiment, after step (c), gas phase etching
is performed on the principal surface of the silicon carbide single
crystal substrate.
[0016] In a preferred embodiment, the surface roughness Ra of the
principal surface of the silicon carbide single crystal substrate
is not less than 0.1 nm and not more than 2.0 nm.
[0017] A silicon carbide single crystal substrate of the present
invention has an identification mark on a principal surface of the
silicon carbide single crystal substrate, the identification mark
being constituted of one or more grooves, wherein a width of the
groove is not less than 50 .mu.m and less than 0.5 mm, and a depth
of the groove is not less than 20 .mu.m, and a surface roughness Ra
of an internal surface of the groove is not more than 1 .mu.m.
[0018] In a preferred embodiment, a bottom surface of the groove is
a solidified surface.
[0019] In a preferred embodiment, the bottom surface of the groove
has a striped pattern.
Advantageous Effects of Invention
[0020] According to the present invention, a silicon carbide single
crystal substrate can be obtained that has an identification mark
on which there is substantially no contaminant in a groove and
which has excellent identifiability.
BRIEF DESCRIPTION OF DRAWINGS
[0021] FIG. 1 is a schematic diagram showing a silicon carbide
single crystal substrate in which an identification mark has been
formed by a method of the present invention.
[0022] FIG. 2 is a flowchart illustrating an embodiment of a method
for forming an identification mark on a silicon carbide single
crystal substrate according to the present invention.
[0023] FIG. 3(a) shows a scanning pattern of a laser beam in a
rough laser step. FIG. 3(b) schematically shows a cross section of
a groove formed in a principal surface of a silicon carbide single
crystal substrate in the rough laser step.
[0024] FIG. 4(a) shows a scanning pattern of a laser beam in a
finishing laser step. FIG. 4(b) schematically shows a cross section
of a groove formed in a principal surface of a silicon carbide
single crystal substrate in the finishing laser step.
[0025] FIGS. 5(a) and 5(b) are a schematic plan view and a
schematic cross-sectional view of a groove formed in a
mechanically-polished principal surface of a silicon carbide single
crystal substrate.
[0026] FIG. 6 is a SEM image of a groove of an identification mark
formed by the method of Example 3.
[0027] FIGS. 7(a) and 7(b) are a surface profile along the
cross-sectional direction and a surface profile along the
longitudinal direction of a groove which constitutes an
identification mark which is formed by the method of Example 3.
[0028] FIG. 8 is a SEM image of a groove of an identification mark
formed by the method of Comparative Example.
[0029] FIGS. 9(a) and 9(b) are a surface profile along the
cross-sectional direction and a surface profile along the
longitudinal direction of a groove which constitutes an
identification mark which is formed by the method of Comparative
Example.
DESCRIPTION OF EMBODIMENTS
[0030] Hereinafter, an embodiment of a method for forming an
identification mark on a silicon carbide single crystal substrate
is described with reference to the drawings. FIG. schematically
shows a silicon carbide single crystal substrate 10 in which an
identification mark 14 is formed by a method for forming an
identification mark on a silicon carbide single crystal substrate
according to the present embodiment. The silicon carbide single
crystal substrate 10 is made of silicon carbide monocrystal. The
polytype of the silicon carbide monocrystal is not particularly
limited. It may be any polytype of silicon carbide monocrystal. The
size and thickness of the silicon carbide single crystal substrate
10 are not particularly limited.
[0031] The silicon carbide single crystal substrate 10 has a pair
of principal surfaces 10a and 10b. The identification mark 14 is
formed in one principal surface 10a. The plane orientation of the
principal surfaces 10a and 10b is not particularly limited. The
crystal axis of the silicon carbide monocrystal and the normal
lines of the principal surfaces 10a and 10b may be identical with
each other (so-called "just substrate"). Alternatively, the normal
lines of the principal surfaces 10a and 10b may form an angle which
is greater than 0.degree. with respect to the crystal axis of the
silicon carbide monocrystal (so-called "off substrate"). The
principal surface 10a that has the identification mark 14 is the
rear surface, while the principal surface 10b is the front surface
on which a semiconductor device is to be formed.
[0032] The principal surface 10b of the silicon carbide single
crystal substrate 10 is preferably a mirror surface. Specifically,
the surface roughness of the principal surface 10b, Ra, is
preferably not more than 2.0 nm. This is because a high-quality
silicon carbide layer or gallium nitride layer is epitaxially grown
on the principal surface 10b for fabrication of a semiconductor
device. The lower limit of the surface roughness Ra of the
principal surface 10b is not particularly limited. However, as the
surface roughness Ra decreases, the processing of the principal
surface 10b requires a longer time, so that the productivity of the
silicon carbide single crystal substrate 10 deteriorates. Thus,
from the viewpoint of industrial mass productivity, the surface
roughness Ra of the principal surface 10b is preferably not less
than 0.1 nm.
[0033] On the other hand, the principal surface 10a has a surface
roughness which is selected according to its use or specifications
required of the silicon carbide single crystal substrate 10.
Specifically, the principal surface 10a may be a mirror surface or
may be a surface finished by mechanical polishing. When the
principal surface 10a is a mirror surface, the surface roughness Ra
of the principal surface 10a is not more than 2.0 nm. When the
principal surface 10a is a surface finished by mechanical
polishing, the surface roughness Ra of the principal surface 10a is
not less than 50 nm and not more than 1000 nm.
[0034] In the present embodiment, the identification mark 14 is
formed in the vicinity of an orientation flat 12 of the silicon
carbide single crystal substrate 10. However, the position of the
identification mark 14 is not particularly limited. The
identification mark 14 may be formed at any other position over the
principal surface 10a.
[0035] The identification mark 14 may be constituted of characters
which are used in various languages, such as numerals, alphabets,
Katakana characters, Hiragana characters, Kanji characters, etc.,
and symbols. The number of characters is not particularly limited.
The identification mark 14 preferably has a size which is
perceivable by a naked eye. For example, it is preferred that the
size of a single character is 0.8 mm or 1.6 mm. The upper limit of
the size of a single character of the identification mark 14 is not
particularly limited. However, when the size of a single character
is excessively large, formation of characters takes a long time.
The groove width is preferably not more than 0.5 mm.
[0036] As will be described in detail hereinbelow, the
aforementioned alphanumeric characters which constitute the
identification mark 14 are not an identification mark which is
constituted of recessed portions in the form of dots but an
identification mark which is constituted of linear grooves. To
secure sufficient visibility for a naked eye, the depth of the
groove is preferably not less than 20 .mu.m, and the width of the
groove is preferably not less than 50 .mu.m.
[0037] Hereinafter, a method for forming an identification mark on
a silicon carbide single crystal substrate according to the present
embodiment is described in detail with reference to FIG. 1 and the
flowchart shown in FIG. 2.
[0038] First, a silicon carbide single crystal substrate 10 is
provided (step S11). As described above, the size, thickness, and
polytype of the silicon carbide single crystal substrate 10 and the
directions of the normal lines of the principal surface 10a and the
principal surface 10b are not particularly limited. The principal
surface 10b of the silicon carbide single crystal substrate 10
before formation of the identification mark 14 may have a surface
roughness which is obtained after being finished by mechanical
polishing or may be a mirror surface.
[0039] On the other hand, the principal surface 10a preferably has
a surface roughness which is obtained after being finished by
mechanical polishing. This is because, when the surface roughness
of the principal surface 10a is generally equal to a surface
roughness which is obtained after being finished by mechanical
polishing, the laser beam for formation of the identification mark
14 is prevented from passing through the silicon carbide single
crystal substrate 10 as compared with the case where the principal
surface 10a is a mirror surface, so that energy can be efficiently
supplied to the principal surface 10a of the silicon carbide single
crystal substrate 10, and the groove of the identification mark 14
can be formed. Further, the principal surface 10a of the silicon
carbide single crystal substrate 10 may be directly irradiated with
a laser beam such that the energy of the laser beam can be supplied
to the principal surface 10a, without providing an energy absorbing
layer on the principal surface 10a of the silicon carbide single
crystal substrate 10 for absorbing the energy of the laser beam.
Further, when the principal surface 10a has a mirror surface before
formation of the identification mark 14, the principal surface 10a
that is a mirror surface is mechanically polished after formation
of the identification mark 14 as will be described later, and
therefore, the previous mirror finishing step is of no use. In view
of such circumstances, specifically, the surface roughness Ra of
the principal surface 10a is preferably not less than 50 nm and not
more than 1000 nm. To sufficiently obtain the above-described
effects, more preferably, the surface roughness Ra of the principal
surface 10a is not less than 100 nm and not more than 500 nm.
[0040] Then, the principal surface 10a of the provided silicon
carbide single crystal substrate 10 is scanned with a laser beam
such that the identification mark 14 is formed in the principal
surface 10a. Formation of the identification mark is realized by
forming the identification mark 14 which is constituted of one or
more grooves by the rough laser process (step S12) and finishing
the inside of the grooves by the finishing laser process (step
S13). First, the rough laser process (step S12) is described.
[0041] As the laser light source which emits a laser beam for
formation of the identification mark 14, a various types of laser
light sources for use in laser marking may be used. Here, the laser
light source may include not only a light-emitting light source
which emits laser light but also an optical system which is used
for adapting the beam diameter, a Q switch which is used for pulse
driving of a laser beam, and a wavelength converter element which
is used for adapting the wavelength of a laser beam. The laser
light source which is used in the present embodiment is configured
to emit a laser beam at a wavelength which is suitable to melting
and evaporation of the silicon carbide monocrystal. Specifically,
the laser light source preferably emits a laser beam at a
wavelength of not less than 532 nm and not more than 1064 nm. A
laser light source which is configured to emit a laser beam at a
wavelength shorter than 532 nm includes an expensive oscillator and
is a large-size device. Therefore, particularly, the cost of
forming an identification mark is likely to increase.
[0042] The beam diameter of the laser beam emitted from the laser
light source depends on the size of the identification mark 14
which is to be formed and the power of the laser light source. For
example, the laser light source emits a laser beam which has a beam
diameter of, for example, not less than 5 .mu.m and not more than
50 .mu.m. The power of the laser light source is, for example, not
less than 1.0 W and not more than 2.0 W. Using a laser light source
whose power exceeds the upper limit is not preferred because there
is a probability that damage, such as a slip, is caused in
crystal.
[0043] The principal surface 10a of the silicon carbide single
crystal substrate 10 is scanned with a laser beam using a laser
light source such that the identification mark 14 is formed in the
principal surface 10a. The principal surface 10a is scanned at the
first energy density such that a groove is formed in the surface
10a of the silicon carbide single crystal substrate 10. FIG. 3(a)
is a plan view schematically illustrating scanning with a laser
beam. A pulsed laser beam is emitted from the laser light source to
irradiate the principal surface 10a with every single pulse of the
laser beam which is represented by a beam spot 22 with the beam
diameter R1 as shown in FIG. 3(a). To irradiate the principal
surface 10a of the silicon carbide single crystal substrate 10 with
the laser beam at a high energy density, the principal surface 10a
is preferably irradiated with the laser beam such that beam spots
22 each of which is formed by a single pulse successively overlap.
As the overlapping area of the beam spots 22 increases, heat can be
applied to the principal surface 10a at a higher energy density. In
this process, a groove 16 which has a cross section such as shown
in FIG. 3(b) is formed in the principal surface 10a of the silicon
carbide single crystal substrate 10.
[0044] To form an identification mark 14 such that it is readily
perceivable by a naked eye, the width of the groove 16 which
constitutes the identification mark 14 is preferably not less than
50 .mu.m, and the depth of the groove 16 is preferably not less
than 20 .mu.m. Usually, the beam diameter of the laser beam is
about several micrometers, which is smaller than the preferred
groove width. Therefore, it is preferred to form a groove which has
a wider groove than the beam diameter of the laser beam by moving
the laser beam for scanning in the extending direction of the
groove 16 while the laser beam is also moved for scanning in a
direction which is not parallel to the extending direction of the
groove 16. Specifically, it is preferred to perform scanning
according to a scanning pattern 24 which is zigzagged with respect
to the extending direction of the groove 16. When scanning is
performed with the laser beam with the kerf width W2, the groove 16
with the width W1 is formed.
[0045] By the laser beam irradiation, the silicon carbide
monocrystal is melted to a predetermined depth from the principal
surface 10a of the silicon carbide single crystal substrate 10, and
the melted silicon carbide monocrystal partially evaporates. Part
of the melted silicon carbide which has not been evaporated then
solidifies. As a result, the groove 16 which constitutes the
identification mark 14 is formed in the principal surface 10a of
the silicon carbide single crystal substrate 10.
[0046] As shown in FIG. 3(b), the internal surface 16a of the
groove 16 formed in the principal surface of the silicon carbide
single crystal substrate 10 is formed by solidification of the
melted silicon carbide monocrystal. Also, minute solidified
substances 18 of the solidified silicon carbide are attached onto
the internal surface 16a. On the principal surface 10a extending
outside the groove 16, there are also solidified substances or a
bump 19 formed by solidification.
[0047] The solidified substances 18 produced inside the groove 16
and the solidified substances or bump 19 produced outside the
groove 16 separate from the silicon carbide single crystal
substrate 10 and attach to the principal surface 10a and the
principal surface 10b of the silicon carbide single crystal
substrate 10 as contaminants so that they can cause adverse
effects, become the cause of scratches in the principal surface 10a
or the principal surface 10b, or turn to dust to become the cause
of contamination of other substrates or contamination inside the
semiconductor device, in a subsequent step for fabrication of the
silicon carbide single crystal substrate 10 or a manufacture step
for manufacturing a semiconductor device using the completed
silicon carbide single crystal substrate 10. For example, when an
abrasive agent which is for use in the subsequent step that is for
fabrication of the silicon carbide single crystal substrate 10 is
brought into the groove 16, the abrasive agent is trapped by the
internal surface 16a because the surface roughness of the internal
surface 16a of the groove 16 is large, so that there is a
probability that the abrasive agent cannot be removed from the
groove 16 even by washing.
[0048] In the method for forming an identification mark on the
silicon carbide single crystal substrate 10 according to the
present embodiment, the finishing laser process (step S13) is
performed, after the rough laser process, on the groove 16 which
has been formed by the rough laser process in order to solve the
above problems. By the finishing laser process, the solidified
substances 18 and the bump 19 that have previously been described
are again melted and evaporated such that the solidified substances
18 and the bump 19 are removed. Further, the internal surface 16a
is melted and solidified so as to have a smooth internal surface.
The solidified substances 18, the bump 19, and the internal surface
16a are formed by solidification of melted silicon carbide
monocrystal, so that they are amorphous or have low crystallinity.
Further, in the rough laser process, carbon or silicon selectively
evaporates, so that solidified substances 18, the bump 19, and the
internal surface 16a have a composition in which silicon or carbon
is excessively contained or a composition in which silicon or
carbon is bound to oxygen. These can be melted and evaporated even
when the applied energy is not as large as the first energy density
in the rough laser process.
[0049] For example, where the processing energy is estimated by a
product of the melting point and the thermal conductivity and the
processing energy of silicon carbide is 1, the processing energy of
Si is about 0.2 and the processing energy of SiO.sub.2 is about
0.02. Thus, by scanning the inside of the groove 16 which has been
formed in the surface 10a of the silicon carbide single crystal
substrate 10 at the second energy density that is lower than the
first energy density, the solidified substances 18 and the bump 19
can be removed, and also, the internal surface 16a can be smoothed.
Further, since the laser beam irradiation is performed at an energy
density which is lower than the first energy density, a portion
extending outside the groove 16 which is made of silicon carbide
monocrystal would not be newly melted or evaporated. That is, the
laser beam irradiation is preferably performed at the second energy
density such that the silicon carbide monocrystal is not melted or
evaporated. Thus, production of new solidified substances 18 or
bump 19 by the finishing laser process is prevented. It is
preferred that the ratio of the total energy of the finishing laser
process to the total energy of the rough laser process is about not
less than 10% and not more than 40%.
[0050] A pulsed laser beam is emitted from the laser light source
to irradiate the inside of the groove 16 formed in the principal
surface 10a with every single pulse of the laser beam which is
represented by a beam spot 22' with the beam diameter R1 as shown
in FIG. 4(a). In FIG. 4(a), the positions of the beam spot 22 in
the rough laser process are shown by broken lines. As seen from
FIG. 4(a), the overlapping area of the beam spots 22' is smaller
than that of the beam spots 22 so that the second energy density is
smaller than the first energy density. The other methods for
decreasing the second energy density include decreasing the laser
power and increasing the scanning speed.
[0051] Preferably as shown in FIG. 4(a), the scanning direction of
the laser beam in the step of the rough laser process and the
scanning direction of the laser beam in the finishing laser process
are different from each other. With this arrangement, the
unevenness in the internal surface 16a of the groove 16 which
depends on the scanning direction of the laser beam in the step of
the rough laser process is flattened in the finishing laser
process, so that the inside of the groove 16 can be further
smoothed. In the present embodiment, the inside of the groove 16 is
scanned with the laser beam according to a scanning pattern 24'
which is parallel to the extending direction of the groove 16. As
shown in FIG. 4(a), it is preferred that a portion extending
outside the groove 16 is also irradiated with the beam spots 22' in
order to remove the bump 19 which has been produced outside the
groove 16 of the principal surface 10a. With this arrangement, the
wall surface of the groove 16 is also irradiated with the laser
beam at a sufficient energy density, whereby the solidified
substances 18 attached to the plane surface are removed, and the
wall surface is smoothed. In other words, it is preferred that a
region which is irradiated with the laser beam by the finishing
laser process entirely includes a region which is irradiated with
the laser beam in the step of the rough laser process and is also
wider than a region which is irradiated with the laser beam in the
rough laser process. Where a region which is irradiated with the
laser beam in the finishing laser process and a region which is
irradiated with the laser beam in the rough laser process are
respectively referred to as the first region and the second region,
the area of the second region is preferably 100% or more of the
area of the first region. To remove the bump 19 which is produced
outside the groove 16, the area of the second region is preferably
110% or more of the area of the first region.
[0052] As shown in FIG. 4(b), by the finishing laser process, the
solidified substances 18 which have been formed inside the groove
16 are removed, and the internal surface 16b of the groove 16 is
further smoothed. The bump 19 which has been produced outside the
groove 16 of the principal surface 10a is also removed. As a result
of the finishing laser process, the surface roughness Ra is not
more than 1 .mu.m at least at the bottom surface of the internal
surface 16b of the groove 16.
[0053] The finishing laser process may be performed after the
entirety of the identification mark 14 is formed by the rough laser
process and the rough laser process is completed, or may be
performed on parts of the identification mark 14 which have
undergone the rough laser process one after another. In this case,
the rough laser process and the finishing laser process are
concurrently performed, and it is therefore preferred to provide a
laser light source for the rough laser process and another laser
light source for the finishing laser process. Where the pulse
interval of the laser beam in the rough laser process is t and the
interval between the rough laser process and the finishing laser
process which are performed in an identical region of the
identification mark 14 is T, T is sufficiently longer than t. That
is, T>>t, so that part of the silicon carbide monocrystal
which is melted by the rough laser process is solidified and
sufficiently cooled before the finishing laser process.
[0054] Formation of the identification mark 14 in the silicon
carbide single crystal substrate 10 may be completed through the
above-described processes. Alternatively, finishing of the
identification mark 14 or finishing of the principal surface 10a in
which the identification mark 14 is provided may be performed. When
mechanical polishing is performed on the principal surface 10a of
the silicon carbide single crystal substrate 10 in which the
identification mark 14 has been formed such that the surface
roughness of the principal surface 10a is reduced, when there is a
minute bump 19' remaining outside the groove 16 of the principal
surface 10a and the bump 19' is to be removed, or when the surface
roughness of the internal surface 16b of the groove 16 is further
reduced, mechanical polishing is performed on the principal surface
10a of the silicon carbide single crystal substrate 10 (step S14).
Specifically, the principal surface 10a of the silicon carbide
single crystal substrate 10 is mechanically polished using a metal
surface plate and an abrasive agent. In this process, the abrasive
agent enters the inside of the groove 16 so that the internal
surface 16b of the groove 16 is also polished with the abrasive
agent. As a result, as shown in FIGS. 5(a) and 5(b), the silicon
carbide single crystal substrate 10 is obtained in which the
surface roughness of the internal surface 16b' is small and which
has a principal surface 10a' with a reduced surface roughness.
Further, the bump 19' remaining outside the groove 16 also can be
removed by this process.
[0055] To remove a damage layer which is formed over the surface of
the principal surface 10a of the silicon carbide single crystal
substrate 10 due to the laser beam irradiation or solidification of
melted silicon carbide, gas phase etching of the principal surface
10a may be performed (step S15). Examples of the etching by the gas
phase method which can be employed in the present embodiment
include ion etching, sputter etching, reactive ion etching, plasma
etching, reactive ion beam etching, and ion beam etching. Any other
gas phase etching method may be employed.
[0056] The type of the gas used in the gas phase etching is not
particularly limited. However, it is preferred to use a gas which
contains fluorine, such as carbon tetrafluoride or sulfur
hexafluoride or a gas which has reactivity with silicon carbide,
such as hydrogen. Further, oxygen may be added in order to enhance
oxidation. The conditions for the etching, such as the power to
apply, depend on the apparatus used for the etching, for example.
The etching rate preferably does not exceed 10 .mu.m/h. If the
etching rate exceeds 10 .mu.m/h, the etching conditions would be
excessively intense for the principal surface 10a of the silicon
carbide single crystal substrate 10 so that, probably, the
principal surface 10a is damaged by ion collision or the surface
morphology of the principal surface 10a after the etching
deteriorates. Thus, this excessive etching rate is not
preferred.
[0057] Since the thickness of the damage layer is small, the
etching that is based on the gas phase method does not need to be
performed for a long period of time. In the gas phase etching, the
etching progresses generally uniformly so that the surface
roughness of the principal surface 10a scarcely varies. Thus, a
principal surface can be obtained in which the surface roughness of
the principal surface 10a before the gas phase etching is generally
maintained and from which the damage layer has been removed.
[0058] If the principal surface 10b of the silicon carbide single
crystal substrate 10 is a surface which is finished by mechanical
polishing in formation of the above-described identification mark,
mechanical polishing and mirror polishing may be performed on the
principal surface 10b after the formation of the identification
mark.
[0059] By the method for forming an identification mark on a
silicon carbide single crystal substrate according to the present
embodiment, a silicon carbide single crystal substrate 10 with an
identification mark 14 is obtained that is constituted of a groove
16 which has such a depth and a width that excellent visibility is
achieved. Solidified substances 18 and the like are scarcely
remaining in the groove 16 which constitutes the identification
mark 14, and at least the bottom surface of the internal surface
16a has a surface roughness of not more than 1 .mu.m. Therefore,
even when mechanical polishing, CMP (chemical mechanical
polishing), or the like, is further performed on the principal
surface 10a in a subsequent step, the solidified substances 18 are
prevented from separating from the groove and causing scratches in
polishing of the principal surface 10a. Also, since the internal
surface 16b of the groove 16 is smooth, the abrasive agent would
not enter or reside in the groove 16. Thus, an excellent silicon
carbide single crystal substrate 10 is obtained in which occurrence
of various problems which are attributed to formation of the
identification mark 14 is prevented in the process of fabricating
the silicon carbide single crystal substrate 10 or in the process
of manufacturing a semiconductor device.
EXAMPLES
[0060] Hereinafter, an example of formation of an identification
mark on a silicon carbide single crystal substrate with the use of
a method for forming an identification mark on a silicon carbide
single crystal substrate according to the present embodiment is
described.
[0061] A 4H silicon carbide single crystal substrate with a
diameter of 3 inches was provided. The surface roughness Ra of the
principal surface in which an identification mark was to be formed
was 0.3 .mu.m. The laser light source used was a Nd:YAG laser
(wavelength: 1064 nm, power: 1.5 W) manufactured by ESI, Inc. This
laser light source had a Q-switch and was used to perform the rough
laser process and the finishing laser process under the conditions
of Examples 1, 2, and 3 as shown in Table 1 such that an
identification mark of nine characters was formed. In Comparative
Example, only the rough laser process was performed for formation
of an identification mark. In Table 1, the kerf width refers to W2
of FIG. 3. When the kerf width was 0, the principal surface of the
substrate was scanned with a laser beam along a groove to be
formed. For example, when the width of the groove was about three
times the diameter of the laser beam, the principal surface was
scanned with the laser beam such that spots in the left column
shown in FIG. 3 were drawn, the principal surface was then scanned
with the laser beam such that spots in the center column were
drawn, and lastly, the principal surface was scanned with the laser
beam such that spots in the right column were drawn. In Table 1,
the energy density is a relative value which was determined with
respect to the energy density of Comparative Example which was
assumed as 100. The total energy refers to a total energy which was
supplied to the substrate by laser beam irradiation for marking a
straight line of 1 mm in each of the rough laser process and the
finishing laser process. The energy ratio refers to the ratio of
the total energy of the finishing laser process to the total energy
of the rough laser process.
[0062] After the formation of the identification mark by the laser,
the principal surface in which the identification mark was formed
was subjected to mechanical polishing with the use of a diamond
slurry in which diamond particles with the average particle
diameter of 5 .mu.m were contained as the abrasive agent.
[0063] After the mechanical polishing, the inside of the groove
constituting the identification mark was observed with an optical
microscope to check whether there was an attached substance, such
as a solidified substance, on the bottom surface and the lateral
surfaces of the groove. Further, the depth and the width of the
groove were measured using an optical length-measuring microscope.
The measurement was performed at an arbitrary position in the
groove, where the groove width on the substrate surface and the
groove depth from the substrate surface were measured. The
measurement was carried out at one arbitrary position in each of
the grooves of three out of nine characters. Further, the surface
roughness Ra of the bottom surface of the groove was measured using
the optical interference type surface roughness measuring apparatus
HD-2000 manufactured by Veeco Instruments Inc. The measurement was
performed on a central portion at an arbitrary position in the
groove, along the line direction (the longitudinal direction of the
groove), in the length of about 0.2 mm. The measurement was carried
out along one arbitrary line in each of the grooves of three out of
nine characters. The results are shown in Table 2.
TABLE-US-00001 TABLE 1 Exam- Exam- Exam- Compar- ple ple ple ative
Ex- 1 2 3 ample Rough Q rate (Hz) 7000 500 500 3000 Laser Power (%)
100 100 100 100 Process Speed (mm/s) 40 10 15 8 Kerf Width 0 0.08
0.08 0.15 (mm) Energy Density 70 75 72 100 Number of 3 1 1 1
Scanning Cycles Total Energy 788 1200 1200 4200 (W) Finish Q rate
(Hz) 500 500 500 None Laser Power (%) 100 100 100 Process Speed
(mm/s) 10 10 10 Kerf Width 0 0 0 (mm) Energy Density 10 10 10
Number of 3 3 5 Scanning Cycles (with offset) Total Energy 225 225
375 (W) Energy Ratio 29 19 31 (%)
TABLE-US-00002 TABLE 2 Exam- Exam- Exam- Compar- ple ple ple ative
Ex- Evaluated Items 1 2 3 ample Attached Substance Groove No No No
Yes Inside Groove Bottom Wall No Yes No Yes Surface Groove Depth
(.mu.m) Before 45 50 50 60 Polish Groove Width (.mu.m) Before 45
100 110 150-170 Polish After 30 80 80 170 Polish Surface Roughness
of Groove 0.4-0.6 0.4-0.6 0.4-0.6 5.0-7.0 Bottom Surface Ra
(.mu.m)
[0064] As seen from Table 2, no attached substance was found at the
bottom surface of the groove in either of Examples 1, 2, and 3. In
Examples 1 and 3, no attached substance was also found at the wall
surface of the groove. On the other hand, in Comparative Example,
attached substances were found at the bottom surface and the wall
surface of the groove. This is probably because, in the methods of
Examples 1, 2, and 3, solidified substances in the groove were
removed by the finishing laser process. It was found from the
results of Examples 1, 2, and 3 that attached substances can be
entirely removed so long as the energy of the finishing laser
process is approximately not less than 19% and not more than 31% of
that of the rough laser process. It is understood that, when a
margin of about 10% is considered, the energy ratio only needs to
be approximately not less than 10% and not more than 40%.
[0065] In Example 2, the reason why there was an attached substance
on the wall surface is probably that the laser beam of the
finishing laser process failed to irradiate the wall surface inside
the groove with a sufficient intensity. On the other hand, in
Example 3, the finishing laser process was performed through five
cycles, and in each scanning cycle, the position of the beam was
offset by 0.02 mm. Therefore, it is inferred that the beam of the
finishing laser process successfully uniformly irradiated the
entire surface inside the groove.
[0066] In each of Examples 1, 2, and 3, the surface roughness Ra of
the bottom surface of the groove of the formed identification mark
was in the range of 0.4 .mu.m to 0.6 .mu.m. On the other hand, in
Comparative Example, the surface roughness Ra of the bottom surface
of the groove was in the range of 5.0 .mu.m to 7.0 .mu.m. It was
found from this result that the surface roughness of the internal
surface of the groove of the identification mark which was formed
according to the methods of Examples 1, 2, and 3 was improved to
about 1/10 of the surface roughness of the groove which was formed
according to the conventional method. In Examples 1, 2, and 3 and
Comparative Example, reduction of the surface roughness Ra by the
mechanical polishing is estimated at about 50 nm to 100 nm, and
therefore, the above-described difference in surface roughness Ra
is not attributed to the mechanical polishing which is performed
after the formation of the identification mark. In Examples 1, 2,
and 3, it can be said that the surface roughness Ra of the bottom
surface of the internal surface of the groove of the identification
mark before the mechanical polishing is at least not more than 1
.mu.m.
[0067] It was confirmed that the identification marks of Examples
1, 2, and 3 had improved visibility for a naked eye as compared
with Comparative Example. It was also confirmed that the
identification marks which were formed by the methods of Examples 2
and 3 had further improved visibility for a naked eye as compared
with the identification mark which was formed by the method of
Example 1.
[0068] FIG. 6 is an enlarged SEM image showing a portion of a
groove of an identification mark which was formed by the method of
Example 3. As seen from FIG. 6, there was substantially no
contaminant on the bottom surface and the lateral surfaces of the
groove. Also, there was substantially no unevenness in the internal
surface of the groove, and it is appreciated that the surface
roughness of the internal surface was very small. Particularly, it
can be seen that the bottom surface of the groove was a solidified
surface, and it had a striped pattern which was generally
perpendicular to the extending direction of the groove. This is
probably because a solidified substance was removed by the
finishing laser process, and as a result, traces were formed at the
bottom surface of the groove due to sequential melting and
solidification of silicon carbide monocrystal along the traveling
direction of the beam spot in the rough laser process, i.e., the
scanning direction of the laser beam. Formation of a solidified
surface over the internal surface of the groove prevented minute
contaminants from remaining on the surface.
[0069] It is also seen that edges which defined the groove were
also sharp, and the principal surface extending outside the groove
was flat. It was confirmed that an identification mark having a
desired shape was also formed by the method of Example 1. In view
of these circumstances, we consider that the identification mark
formation methods of Examples 1 and 3 are more preferred among
Examples 1, 2, and 3.
[0070] FIGS. 7(a) and 7(b) respectively show a surface profile
along a direction perpendicular to the extending direction of a
groove of an identification mark formed by the method of Example 3
and a surface profile along the extending direction of that groove.
As seen from these graphs, of the internal surface of the groove,
at least the bottom surface had a surface roughness Ra of not more
than 1 .mu.m.
[0071] FIG. 8 is an enlarged SEM image showing a portion of a
groove of an identification mark which was formed by the method of
Comparative Example. As seen from FIG. 8, there were a large number
of small solidified substances attached onto the internal surface
of the groove so that the internal surface of the groove had an
uneven shape. Also, the principal surface extending outside the
groove was not flat but had bumps. FIGS. 9(a) and 9(b) respectively
show a surface profile along a direction perpendicular to the
extending direction of a groove of an identification mark formed by
the method of Comparative Example and a surface profile along the
extending direction of that groove. As seen from these graphs, the
internal surface of the groove had a surface roughness Ra of not
less than several tens of micrometers, so that the internal surface
of the groove was not smooth.
[0072] From the above results, it was found that an identification
mark constituted of a groove which has no contaminant attached onto
the internal surface and of which the internal surface is very
smooth can be formed by the methods of Examples in which the rough
laser process and the finishing laser process are performed.
Employing a mark which is in the form of a groove rather than dots
contributes to excellent identifiability. It was found that, from
the viewpoint of visibility, a kerf width is provided, and the
scanning pattern of the laser beam is zigzagged in such a manner
that a groove width of not less than about 50 .mu.m is secured,
whereby an identification mark with excellent visibility can be
formed.
INDUSTRIAL APPLICABILITY
[0073] The present invention is suitably applicable to a silicon
carbide single crystal substrate which is used in various uses,
including manufacture of a semiconductor device.
REFERENCE SIGNS LIST
[0074] 10 silicon carbide single crystal substrate [0075] 10a, 10b
principal surface [0076] 12 orientation flat [0077] 14
identification mark [0078] 16 groove [0079] 18 solidified substance
[0080] 19 bump [0081] 22 beam spot [0082] 24 scanning pattern
* * * * *